The present invention relates to the PTC thermistors in which a conductive polymer material having a positive temperature coefficient (PTC) of resistance is employed, and methods for manufacturing the same.
PTC thermistors have been commonly used in self-regulating heaters, and are increasingly employed in electronic devices as components to protect against overcurrent. Exposure to overcurrent in an electric circuit causes the conductive polymer sheet inside a PTC thermistor to heat up and expand. This thermal expansion of the conductive polymer sheet increases the resistance of the PTC thermistor and thus reduces the current to a safer level. There are, increasing demands for PTC thermistors which carry high currents, have low resistance, are compact in size, and yield a low voltage drop.
A conventional PTC thermistor is described below.
One known PTC thermistor is disclosed in the Japanese Laid-open Patent No. S61-10203. This PTC thermistor is created by laminating a plurality of alternate layers of conductive polymer sheets and metal foils, with side electrodes on opposing sides.
FIG. 10 is a sectional view of a conventional PTC thermistor. In FIG. 10, a conductive polymer sheet 1 is made of a high polymer material, such as cross-linked polyethylene, and dispersed conductive particles, such as carbon black. An inner electrode 2 is made typically of a sheet of metal foil, and is sandwiched between the conductive polymer sheets 1. The inner electrode 2 is also disposed on the top and bottom of the conductive polymer sheet 1, while leaving a no electrode area 3 at the starting end, portions of the middle and finishing ends of the conductive polymer sheet 1 as shown. Alternate layers of the inner electrode 2 and conductive polymer sheet 1 form a laminated body 4. A side electrode layer 5 form a leader section, and is disposed at the side of the laminated body 4 so as to be electrically coupled to one end of the inner electrode 2.
However, the conventional PTC thermistor created by laminating the conductive polymer sheet 1 and inner electrode 2 alternately to create low resistance undergoes repetitive expansion and shrinkage of the conductive polymer sheet 1 when an overcurrent condition is created and alleviated. This may cause failure in connections to the side electrode due to cracking generated as a result of stresses generated by the expansion and contraction of the conductive polymer sheet 1.
The present invention aims to provide a highly reliable PTC thermistor with good withstand voltage which eliminates failure in a connection to a side electrode by cracks, and its manufacturing method.
The PTC thermistor of the present invention comprises:
A laminated body made by alternately laminating a conductive polymer sheet and an inner electrode;
an outer electrode disposed on a top and a bottom of the laminated body; and
a multi-layered side electrode disposed at the center of a side of the laminated body, and electrically coupled with the inner electrode and the outer electrode.
A side of the laminated body has:
i) an area on which the side electrode is disposed and
ii) an area on which the side electrode is not disposed.
In a method for manufacturing the PTC thermistor of the present invention, the conductive polymer sheet is sandwiched from the top and the bottom by metal foils and integrated by heat pressing to form the laminated body. The laminated body is then sandwiched from the top and the bottom by other conductive polymer sheets, and the laminated body and the conductive polymer sheets are sandwiched from the top and the bottom by the metal foils. They are integrated by heat pressing. These processes are repeated for lamination.
In the PTC thermistor as configured above, a side electrode comprises multiple layers and is disposed at the center of the side of the laminated body so as to be electrically coupled to the inner electrodes and the outer electrodes. In addition, the side of the laminated body has areas with and without the side electrode. This feature reduces mechanical stress in the side electrode at the boundary of the multiple layers of the side electrode layer even when mechanical stress due to thermal impact is applied to the side electrode through repetitive thermal expansion of the conductive polymer sheet during operation of the PTC thermistor. Mechanical stress in the side electrode may also be reduced by extrusion of an expanded conductive polymer sheet to an area where the side electrode is not formed. Thus, generation of cracks by concentrated mechanical stress is avoided, thereby eliminating failure in an electrical connection by cracks. In a method for manufacturing PTC thermistors of the present invention, a process to integrate the laminated body, conductive polymer sheet, and metal foil by heat pressing is repeated for lamination. This process allows uniform thickness of the conductive polymer sheet in each layer to be achieved. Accordingly, a highly reliable PTC thermistor with good withstand voltage is obtained.